1. Technical Field
The embodiments herein generally relate to microelectromechanical systems (MEMS) actuators, and, more particularly to piezoelectric MEMS actuators.
2. Description of the Related Art
MEMS piezoelectric actuators are the basic building blocks for complex electromechanical systems such as radio frequency (RE) MEMS, optical switching, robotics, and many more systems. Typically, piezoelectric actuators outperform electrostatic actuators as they have lower power consumption, smaller size, smaller actuation voltages, and tend to attain greater displacements.
In most conventional piezoelectric actuators, as shown in FIGS. 1(A) and 1(B), the structure includes a structural dielectric layer 104 on top of which the bottom electrode 103 is present. The piezoelectric layer 102 is on top of this bottom electrode 103 and is generally made of lead-zirconate-titanate (PZT), zinc oxide (ZnO), or aluminum nitride (ALN). A top electrode 101 is positioned above the piezoelectric layer 102. The x-y neutral plane of the structure is below the mid-plane of the piezoelectric layer 102 under this configuration. When a voltage is applied between the electrodes 101, 103 a strain is produced which leads to an axial force acting on the mid-plane of the piezoelectric layer 102. This force acts at a perpendicular distance from the neutral axis and results in a bending moment that generates an out-of-plane upward deflection of the entire actuator structure. FIG. 2 illustrates a coordinate system and depiction of neutral planes on the conventional piezoelectric unimorph actuator of FIGS. 1(A) and 1(B).
At small voltages, with polarity opposite that of the poling direction of the ferroelectric/piezoelectric material, a vertical ferroelectric/piezoelectric MEMS actuator will generally deflect downward meaning they provide negative out-of-plane displacement. However, as the voltage increases to a value near the coercive field, the actuator will switch directions and will then bend upwards (out of the wafer plane). As the field strength is increased further, the actuator will continue to bend upward. If the opposite polarity voltage is applied, the actuator will bend upward (out of the wafer plane) for all voltages, Consequently, conventional vertical piezoelectric MEMS unimorph actuators using ferroelectric materials will only provide positive out-of-plane deflections for large operating voltages and are generally unable to attain large negative (out-of-the plane) deflections. There is also a need to have devices capable of positive or negative deflections, at large operating voltages, integrated onto the same substrate within the same batch fabrication MEMS manufacturing process.